1. Technical Field
The present invention relates to a light-emitting device capable of converting electric energy to light, and of emitting light, and particularly to an organic electroluminescent device (“light-emitting device” or “EL device”) which can be used to advantage in fields such as display devices, displays, backlights, light sources for illumination, light sources for recording, light sources for exposure, light sources for reading, signs, signboards, interior decorations and optical communications.
2. Related Art
Organic electroluminescent (EL) devices have been attracting attention as promising display devices for obtaining luminescence of a high luminance at a low driving voltage. An important characteristic value of an organic electroluminescent device is external quantum efficiency. External quantum efficiency is calculated by “external quantum efficiency φ=number of photons discharged from a device/number of electrons injected into a device”. The higher the value of external quantum efficiency, the lower the power consumption of the device, and thereby a device having a higher value of external quantum efficiency is advantageous.
The external quantum efficiency of the organic electroluminescent device is also determined by “external quantum efficiency φ=internal quantum efficiency×light extraction efficiency”. In an organic EL device which uses fluorescence from an organic compound, the limit value of the external quantum efficiency is considered to be about 5% since the limit value of internal quantum efficiency is 25% and light extraction efficiency is about 20%.
A device which uses a triplet light-emitting material (phosphorescent light emitting material) has been reported as a method for improving the external quantum efficiency of the device by enhancing the internal quantum efficiency of the organic electroluminescent device (for instance, see WO 00/70655). In comparison with a conventional device using fluorescence (singlet light-emitting device), this device can enhance external quantum efficiency and a maximum value of external quantum efficiency of 8% has been attained (external quantum efficiency at 100 cd/m2 is 7.5%). However, since a phosphorescent light emission from a heavy atomic metal complex is used, the response of light emission has been slow, and an improvement in durability also remains to be desired.
A singlet light-emitting device that uses energy translation from triplet excitons to singlet excitons has been reported as a method for alleviating this problem (for instance, see WO 01/8230). However, the device described therein has low efficiency, and the color of light emission is imited to red. There is, therefore, a need for a further improvements to the device.
It is accordingly an object of the present invention to provide an organic electroluminescent device which has satisfactory level of light-emitting efficiency, and in particular emits light of a blue color.